ISL3150E, ISL3152E, ISL3153E, ISL3155E, ISL3156E, ISL3158E
Twisted pair is the cable of choice for RS-485/422 networks.
ESD Protection
Twisted pair cables tend to pick up noise and other
electromagnetically induced voltages as common mode
signals, which are effectively rejected by the differential
receivers in these ICs.
All pins on these devices include class 3 (>7kV) Human
Body Model (HBM) ESD protection structures, but the
RS-485 pins (driver outputs and receiver inputs)
incorporate advanced structures allowing them to survive
ESD events in excess of ±16.5kV HBM and ±16.5kV (1/2
duplex) IEC61000-4-2. The RS-485 pins are particularly
vulnerable to ESD strikes because they typically connect to
an exposed port on the exterior of the finished product.
Simply touching the port pins, or connecting a cable, can
cause an ESD event that might destroy unprotected ICs.
These new ESD structures protect the device whether or
not it is powered up, and without degrading the RS-485
common mode range of -7V to +12V. This built-in ESD
protection eliminates the need for board level protection
structures (e.g., transient suppression diodes), and the
associated, undesirable capacitive load they present.
Proper termination is imperative, when using the 20Mbps
devices, to minimize reflections. Short networks using the
115kbps versions need not be terminated, but, terminations
are recommended unless power dissipation is an overriding
concern.
In point-to-point, or point-to-multipoint (single driver on bus)
networks, the main cable should be terminated in its
characteristic impedance (typically 120Ω) at the end farthest
from the driver. In multi-receiver applications, stubs
connecting receivers to the main cable should be kept as
short as possible. Multipoint (multi-driver) systems require
that the main cable be terminated in its characteristic
impedance at both ends. Stubs connecting a transceiver to
the main cable should be kept as short as possible.
IEC61000-4-2 Testing
The IEC61000 test method applies to finished equipment,
rather than to an individual IC. Therefore, the pins most likely
to suffer an ESD event are those that are exposed to the
outside world (the RS-485 pins in this case), and the IC is
tested in its typical application configuration (power applied)
rather than testing each pin-to-pin combination. The
IEC61000 standard’s lower current limiting resistor coupled
with the larger charge storage capacitor yields a test that is
much more severe than the HBM test. The extra ESD
protection built into this device’s RS-485 pins allows the
design of equipment meeting level 4 criteria without the need
for additional board level protection on the RS-485 port.
Built-In Driver Overload Protection
As stated previously, the RS-485 spec requires that drivers
survive worst case bus contentions undamaged. These
devices meet this requirement via driver output short circuit
current limits, and on-chip thermal shutdown circuitry.
The driver output stages incorporate short circuit current
limiting circuitry which ensures that the output current never
exceeds the RS-485 spec, even at the common mode
voltage range extremes.
In the event of a major short circuit condition, devices also
include a thermal shutdown feature that disables the drivers
whenever the die temperature becomes excessive. This
eliminates the power dissipation, allowing the die to cool. The
drivers automatically re-enable after the die temperature
drops about 15 degrees. If the contention persists, the thermal
shutdown/re-enable cycle repeats until the fault is cleared.
Receivers stay operational during thermal shutdown.
AIR-GAP DISCHARGE TEST METHOD
For this test method, a charged probe tip moves toward the
IC pin until the voltage arcs to it. The current waveform
delivered to the IC pin depends on approach speed,
humidity, temperature, etc., so it is difficult to obtain
repeatable results. The ISL315XE 1/2 duplex RS-485 pins
withstand ±16.5kV air-gap discharges.
Low Power Shutdown Mode
These CMOS transceivers all use a fraction of the power
required by their bipolar counterparts, but they also include a
CONTACT DISCHARGE TEST METHOD
shutdown feature that reduces the already low quiescent I
CC
to a 70nA trickle. These devices enter shutdown whenever
the receiver and driver are simultaneously disabled
During the contact discharge test, the probe contacts the
tested pin before the probe tip is energized, thereby
eliminating the variables associated with the air-gap
discharge. The result is a more repeatable and predictable
test, but equipment limits prevent testing devices at voltages
higher than ±9kV. The RS-485 pins of all the ISL315XE
versions survive ±9kV contact discharges.
(RE = V
and DE = GND) for a period of at least 600ns.
CC
Disabling both the driver and the receiver for less than 60ns
guarantees that the transceiver will not enter shutdown.
Note that receiver and driver enable times increase when
the transceiver enables from shutdown. Refer to Notes 7, 8,
9, 10 and 11, at the end of the Electrical Specification table
on page 8, for more information.
Data Rate, Cables, and Terminations
RS-485/422 are intended for network lengths up to 4000’,
but the maximum system data rate decreases as the
transmission length increases. Devices operating at 20Mbps
are limited to lengths less than 100’, while the 115kbps
versions can operate at full data rates with lengths of several
thousand feet.
FN6363.0
December 14, 2006
12
INTERSIL [ Intersil ]
